A better way to say this would be, "You should drive very carefully when the road is icy." Drives is only used when the subject is he, she, it or a specific noun and means an action that is ongoing. It is not used when followed by should. The man drives very carefully when the road is icy. He drives very carefully when the road is icy. He should drive very carefully when the road is icy.

The sentence should be "You should drive carefully when the road is icy." The verb is always in the infinitive form after should/would.

Neither. You should drive carefully when the road is icy. You should always take care when driving whether or not the roads are iced over. The word so is not needed and there is no final s for commands or warnings. The word ice can be used as a verb but the meaning is not as clear. I iced the drinks to keep them cold.